1. Field of the Invention
The present invention relates to a novel steam turbine blade and steam turbine rotor. The present invention also relates to a steam turbine with those blades and rotors, and a power plant with the turbines.
2. Description of the Related Art
FIG. 9 is an oblique perspective view showing a state where known steam turbine blades are attached to a turbine rotor. Each of the turbine blades used in a steam turbine has a blade root 4 formed in a complex shape, and is attached to the turbine rotor by engaging the blade root 4 in a blade groove (or a disc slot in the turbine rotor) 5 having a shape complementary to that of the blade root 4. The blade root 4 has, for example, a structure of the axially-inserted fir tree type. A shroud 1 is provided at a top of the turbine blade for the purpose of reducing the steam leak loss. Hitherto, the shroud 1 has been prepared separately from an airfoil 2 having a blade profile and combined to the airfoil 2 by riveting a tenon 6 formed at the blade top.
Centrifugal forces acting on the whole of the turbine blade are mostly supported at serrations of the blade root 4 and the blade groove 5. In the case of the turbine blade having the blade root 4 of the axially-inserted fir tree type, therefore, high peak stresses are generated due to the centrifugal forces in notches formed in the blade root 4 and the blade groove 5. Accordingly, the notches are the most risky portions with regards to fatigue that is one of turbine damage modes, and decide the life of the entire turbine in some cases.
One solution for overcoming that problem is proposed by Patent Document 1 (JP,A 54-96618). More specifically, as a method for reducing the peak stresses generated in the notches formed in the blade groove 5 and the blade root 4 of the turbine blade having the blade root 4 of the axially-inserted fir tree type, thereby prolonging the fatigue life, Patent Document 1 discloses such a structure that the radius at the bottom of each notch is increased to reduce the peak stress.
FIG. 10 is a plan view of airfoils and platforms forming roots of the airfoils in the structure of a known steam turbine blade as viewed from the outer peripheral side in the radial direction of a turbine rotor. In other words, Patent Document 2 (JP,A 8-260902) discloses such a structure that, as shown in FIG. 10, plan shapes of platforms 9 forming roots of airfoils 2 as viewed in the radial direction of the turbine rotor are inclined relative to the axial direction of the turbine rotor.